The present applicant noticed that, with respect to the construction of a vortex generator, that the components were arranged in parallel at a considerable distance from each other, so as to be opposed to each other vertically in the direction of the flow. That vortex generator, in the vortex flow meter, was described in the published specification of Japanese Patent Publication No. 46-10233/1971 and the effect, based on the mutual structural arrangement of the vortex generators, couldn't be anticipated. Furthermore, the present applicant proposed a vortex flow meter as shown in the published specifications of Japanese Utility Model Publication No. 55-45296/1980, in which two same-shaped and same-sized vortex generators, having a cross section in a triangular shape, are arranged so as to be opposed to each other and the distance therebetween is equal to 0.1d to 0.9d when the width of the vortex generator is d. Namely, by limiting the distance between the vortex generators to the above-mentioned area, it was possible to stabilize the Karman vortex.
The vortex flow meter described in the above-mentioned published specification of Japanese Utility Model Publication No. 55-45296/1980 achieved the stabilization of the Karman vortex, and the characteristic curve of instrumental error was superior. However, since the instrumental error increases slowly in the area of the low flow rate, in order to enlarge the measurement range of the flow rate with a high accuracy, the area of high flow rate needed to be expanded. As a result of it, there existed a need for suppressing the increase of pressure loss.
Furthermore, by combining of such vortex generators, the magnitude of vortex circulation was changed by the action of the mutual interference of the vortexes, depending on the magnitude of the distance between them. The vortex generating frequency of a predetermined condition differed from the frequency of a single vortex generator. In the prior art, the vortex generator was an unsatisfactory one for being installed in a device requiring a flow rate control of quick response, because the vortex generating frequency in a constant flow rate was decreased.
The present applicant proposed a Karman vortex generating device in the Karman vortex flow meter described in the published specification of Japanese Patent Publication No. 55-40804/1980 and in which a Karman vortex generator is installed against the fluid's flow at the most pointed end thereof and a large number of respectively, independent element plates for generating Karman vortexes are installed behind the vortex generator at desired intervals.
The above-mentioned Karman vortex generator has an angular point on the flowing axis of the fluid passage and the cross section thereof is in the shape of an equilateral triangle opposed to the fluid's flow and perpendicular thereto. By providing space chambers capable of developing the vortex generated here (i.e. at the Karman vortex generator) to a most suitable intensity by means of a large number of downstream-side element plates for generating Karman vortex (called "flat plate(s)" simply hereafter), a sufficiently strong vortex can be developed in proportion to the downstream position. In consequence, a stable vortex generator can be created.
In the prior art, when a flat plate is put at the downstream-side of the vortex generator the vortex exerts an amplifying effect. Concerning the number of flat plates, the number required for obtaining a vortex of such intensity, so that the vortex is peeled off, is sufficient. In the case of the vortex being peeled off from the flat plate in the final stages, an adequate of the number is selected according to the optimum conditions needed. The time required for peeling off the vortex is short because a sufficiently developed vortex has already been produced up to then. Furthermore, the vortex can be produced with a remarkably stable vortex frequency. However, there existed a subject matter to be settled with the instrumental error being a plus in the area of the low Reynolds number.
The present applicant proposed a vortex flow meter described in the Japanese Patent Application No. 58-60333/1983 (laying-open No. 59-187222/1984), in which a tubular member is disposed behind a vortex generator in a direction of intersecting it and at least one pair of pressure guiding holes are formed at a predetermined interval in the direction of the tubular member's axis. The present applicant noticed that, in the flow meter as mentioned above, the fluid's variation of flow had been caused by applying a variation to the inside of the tubular member, and the fluid's flow had been rectified by guiding a fluid-flowing variation into the tubular member having a comparatively small circumference for passing therethrough. In such a manner, a turbulence vortex contained in the flow of the fluid in the fluid passage wasn't detected as compared with the case when there is no tubular member. Consequently, a noise component of the detection signal of an excellent S/N characteristic was obtained. The detection of the vortex signal already generated was improved in such a manner as mentioned above. And further, with respect to the vortex generator, an acute-angled equilateral triangular element was disposed of in direction of the fluid's flow and independent flat plates were arranged respectively behind the element, in order, at a desired interval, as shown in the published specification of Japanese Patent Publication No. 55-40804/1980. In such a manner, an amplifying effect was given to the vortex and thereby a strong and stable vortex could be generated.
According to the above-mentioned prior art, a stable vortex flow meter which didn't exist in the past could be realized in combination with a vortex generating means for generating a strong and stable vortex and a detection means for detecting the vortex generated by the vortex generating means having an excellent S/N characteristic. However, in order to perform a wider range of flow rate measurement, it was necessary to get a stronger vortex signal and keep the value of the Strouhal number with a wide range and also constant to correspond to the above-mentioned conditions. Furthermore, the vortex flow meter had a plus instrumental error in low flow rate area, as described in the afore-mentioned published specifications of Japanese Patent Publication No. 55-40804/1980. Therefore, although the vortex generator was stable, it was unsatisfactory for a flow meter requiring a wide range of flow rate measurement.